1. Field of the Invention
This invention relates to the field of electronic storage devices, and in particular to a preamplifier for transferring information from a magnetic medium via magnetic heads.
2. Description of Related Art
Magnetic recording devices are used to write information to, or read information from, a magnetic medium, such as a floppy disc or a hard disc for storing programs and data in a computer.
The read channel for a magnetic recording device includes a sensor in the form of a magneto-resistive head in close proximity with the magnetic medium. When the magnetic material is moved relative to the sensor, a flux is induced in the sensor in dependence upon the local orientation of the magnetic material, thereby generating an information signal that can be amplified and then decoded.
When information is written in digital form, such as for computer data storage or digital recording of music, a current is generated by a write channel and passed through a thin film head in one direction to write a binary xe2x80x9c0xe2x80x9d and in the opposite direction to write a xe2x80x9c1xe2x80x9d. When the medium is read by the sensor, or read head, the portions recorded with a binary xe2x80x9c0xe2x80x9d will induce a current in the head in the one direction and portions recorded with a binary xe2x80x9c1xe2x80x9d will induce a current in the opposite direction, which is then decoded by a bit detector. Because this invention applies to the processing of signals from the read head of a magnetic disk device, write operations are not discussed further herein.
To achieve high density or high speed, or both, multiple heads are often used. A hard disk drive, for example, typically includes multiple magnetic discs, or platters, each side of which is used for reading and writing information. The read heads and write heads are mounted on movable arms positioned at each side of disk. FIG 1 illustrates an example block diagram of the read signal path of a xe2x80x9cpre-ampxe2x80x9d 100 for a disk drive with mulitple read heads 121a-121n. This pre-amp 100 is typically controlled by a microcontroller 50 in the disk drive that processes commands from a higher level system (not shown) for reading, writing, and configuring the disk drive. Each head 121a-n is uniquely addressable by a head selector 110, in combination with a signal (R/Wxe2x80x2) that indicates whether a read head 121a-n or a write head is being accessed. Typically, the head selector selects one of the heads H based on an address provided by the microcontroller 50 on the address bus 102. Each read head 121a-n has an associated xe2x80x9cReader Front Endxe2x80x9d (RFE) 120. The RFE 120 contains an amplifier 125 that provides the read signal from the selected read head 121 to a xe2x80x9cReader Back Endxe2x80x9d (RBE) 130. The RBE 130 further processes the read signal via a reader 140, that includes filters, amplifiers, and the like to reduce noise and other anomalies that are associated with the response of a read head 121 to the magnetic flux on a magnetic medium (not shown) in the vicinity of the read head 121. As illustrated, due to the relatively poor signal to noise ratio that is common in disk drives, differential signaling is used. The buffer 150 provides the differential read signals RDp and RDn to a read/write channel interface device 190 that converts this differential input into a logic value corresponding to the information that is encoded as flux on the magnetic medium in the vicinity of the select read head 121. This logic information is communicated to a microcontroller 50, typically as a collection of logic values forming a byte or word. The communicated information may be, for example, the information bits of a database file, a text or graphics file, an audio or video recording, and so on. Also illustrated in FIG. 1 is an MR bias setting block 160 that is configured to control the amount of bias current that is applied to each read head 121. Bias current is typically provided to magneto-resistive (MR) heads to improve noise immunity and linearity. Also shown in FIG. 1 are control and data signals for writing information to the medium.
FIG. 2 illustrates an example block diagram of a reader back end 130 that illustrates the stages 141-149 that are typically included in a convention reader 140. A conventional reader 140 includes a read multiplexer 141 that segregates the outputs from all of the RFEs 120, and provides some amplification to the differential signal from the selected RFE. Following the read multiplexer 141, the conventional reader 140 provides a bandwidth limiting stage 143, that sets the high frequency pole of the stage 143 based on a bandwidth control input 131. After the read multiplexer and bandwidth control stages, the signal is amplified in a programmable amplifier 145, whose gain is controlled by a gain select input 132. The next stage is a signal suppression stage 147 that suppresses the signal as required during transitions to further optimize the performance of the reader 140. An amplification and high frequency boost stage 149 provides further amplification to the signals, including an increased amplification of high frequency signals, to compensate for poles of the read stages and parasitic effects, as well as to provide the required high frequency bandwidth. Other gain control capabilities, common in the art, are provided to further optimize the performance of the reader back end 130. For example, in a conventional reader 140, a squelch control input 133 attenuates the signal during transitions, such as when a new head is selected, including switching from write heads to read heads. Note that the particular ordering of stages 141-149 is presented for illustrative purposes only, different systems may provide a different ordering of processing, as well as additional or fewer stages as required to achieve a given performance level.
As is known in the art, the various stages in the reader 140 each provide a desired feature and advantage, but at the cost of circuit area and performance. Each stage includes characteristic poles and the combined filter effect of this variety of poles limits the bandwidth of the reader 140. Each stage also typically includes an input component, a transform component, and an output component, each stage""s input component being based on the prior stage""s output component. The transform component effects the desired function of each stage. As the demand for low-cost computer systems continues to increase, the design requirements for hard disk drives, and other magnetic storage devices, are particularly cost-sensitive.
It is an object of this invention to provide a low cost preamplifier for reading data that is contained on a magnetic medium, via a read head. It is a further object of this invention to provide a low-cost preamplifier that is programmable. It is a further object of this invention to provide a low-cost preamplifier that has a high bandwidth. It is further object of this invention to provide a low-cost preamplifier that has a very low write-to-read settling time. It is a further object of this invention to provide a low-cost amplifier that suppresses reader output during transitions.
These objects and others are achieved by providing a reader with a gain stage that is configured to effect multiple functions. In a preferred embodiment, the gain stage is a single stage circuit that includes programmable gain, programmable bandwidth and high-frequency boost, and squelch control. The single stage circuit also includes a folded cascode current drive that provides an increased dynamic range of the gain stage. To provide a low DC offset, the reader also includes an integrator that is operated in closed loop to appropriately attenuate the currents from the cascode current drive. By employing a multi-function single stage reader, a substantial reduction in circuit area, and a substantial increase in bandwidth, is achieved.